Generally, a vehicle wheel which is in contact with a road surface rotates according to a rotation of a steering wheel during traveling or stopping of a vehicle. That is, when the steering wheel rotates to the left or right, the vehicle wheel rotates in the same direction.
However, as the vehicle wheel is in contact with the road surface, there may be a problem that rotation amounts between the steering wheel and the vehicle wheel become different with each other due to a friction generated between the vehicle wheel and the road surface.
To solve this problem, a torque sensor is provided to measure and compensate a deviation in a rotation angle between the steering wheel and the vehicle wheel.
That is, the torque sensor is a device that measures the deviation in a rotation angle between the steering wheel and the vehicle wheel and allows an additional driving means to rotate the vehicle wheel as much as the measured deviation, so that the vehicle can be steered safely and accurately in a direction as intended thus enhancing a steering convenience.
The torque sensor is generally classified into a contact type and a contactless type. However, as the contact type has a problem of a noise generation and an inferior durability, the contactless type has been preferred recently.
The contactless type torque sensor is classified into a magnetic resistance detection type, a magnetic deformation detection type, a capacitance detection type and an optical detection type.
A conventional magnetic resistance detection type torque sensor, which is provided in an electric power steering system, has an input shaft whose upper end is coupled to the steering wheel operated by a driver and an output shaft whose upper end is coupled to a lower end of the input shaft through a torsion bar. A lower end of the output shaft is connected to a vehicle wheel.
The lower end of the input shaft including the torsion bar and the upper end of the output shaft are covered by a housing, which the torque sensor and the driving means described above are also provided in.
In this case, the input shaft has a permanent magnet where a polarity is changed at regular intervals.
Also, the output shaft is provided with a detection ring having a gear structure which corresponds to the polarity number of the permanent magnet and which is made from a ferromagnetic substance that can generate a magnetic induction caused by the permanent magnet provided to the input shaft.
The sensor is provided with a structure to be combined to the detection ring to detect magnetism.
Therefore, a rotation force is transmitted to the input shaft when the driver operates the steering wheel. Due to the rotation of the input shaft, the torsion bar is rotated.
As the torsion bar is coupled to the output shaft, the rotation force is also transmitted to the output shaft. Thus, the vehicle wheel is rotated in a direction in which the steering wheel is operated.
In this case, a relative twist between the permanent magnet provided to the input shaft and the detection ring of the gear structure provided to the output shaft causes a change in an area by which the permanent magnet and the detection ring face each other.
Accordingly, a magnetic flux is changed in the detection ring and the change of the magnetic flux is detected through the sensor, so that a twist angle of the output shaft relative to the input shaft can be detected.
However, as the conventional torque sensor as described above has a structure where the detection ring continually rotates around the sensor, there is a much fluctuation in the magnetic force originated from the permanent magnet, and a stationary detection ring should be additionally installed to avoid an interference between the detection ring and a magnet detection device when rotating.
Also, if the stationary detection ring is additionally installed, the magnetic flux generated from the permanent magnet is magnetically induced firstly through the detection ring and then magnetically induced secondly through the stationary detection ring.
Therefore, the magnetic flux is decreased and distorted by going through the magnetic induction two times, so that the twist between the output shaft and the input shaft cannot be detected correctly.